Communication and radar systems frequently employ coded pulses for the transmission of data and employ receivers having filters matched to the coding. As is well known, such filters are utilized in radar for compressing a relatively long duration transmitted pulse signal into a relatively short duration signal but of increased amplitude. For maximum use of the available spectrum of transmission of the pulse signal, it is desirable to utilize a waveform modulation which uniformly distributes the spectral components of the waveform throughout the signal bandwidth, a desirable spectrum being one similar to the spectrum of noise wherein the spectral energies are uniformly distributed across the available bandwidth.
A problem arises in that signals may be transmitted between stations which are moving relative to each other, or may be reflecting from a target which is moving relative to a transmitter or receiver of the signal. Such motion induces a Doppler frequency shift which may so alter the signal that the filter in the receiver is no longer matched to the signal. In the past, it has been found that a signal modulation which is Doppler tolerant, in the sense that the receiving filter remains substantially matched to the signal in the presence of Doppler frequency shifts, has not had a distribution of spectral energies which is as uniformly distributed across the available transmission bandwidth as would be desired. On the other hand, signals having modulations which do provide the desired spectrum utilization have not been Doppler tolerant.